Astrophys. J., 756, 24 (2012/September-1)
Substellar objects in nearby young clusters (SONYC). VI. The planetary-mass domain of NGC 1333.
SCHOLZ A., JAYAWARDHANA R., MUZIC K., GEERS V., TAMURA M. and TANAKA I.
Abstract (from CDS):
Within the SONYC (Substellar Objects in Nearby Young Clusters) survey, we investigate the frequency of free-floating planetary-mass objects (planemos) in the young cluster NGC 1333. Building upon our extensive previous work, we present spectra for 12 of the faintest candidates from our deep multi-band imaging, plus seven random objects in the same fields, using MOIRCS on Subaru. We confirm seven new sources as young very low mass objects (VLMOs), with Teff of 2400-3100 K and mid-M to early-L spectral types. These objects add to the growing census of VLMOs in NGC 1333, now totaling 58. Three confirmed objects (one found in this study) have masses below 15 MJup, according to evolutionary models, thus are likely planemos. We estimate the total planemo population with 5-15 MJup in NGC 1333 is ≲ 8. The mass spectrum in this cluster is well approximated by dN/dM ∝ M –α, with a single value of α = 0.6±0.1 for M < 0.6 M☉, consistent with other nearby star-forming regions, and requires α ≲ 0.6 in the planemo domain. Our results in NGC 1333, as well as findings in several other clusters by ourselves and others, confirm that the star formation process extends into the planetary-mass domain, at least down to 6 MJup. However, given that planemos are 20-50 times less numerous than stars, their contribution to the object number and mass budget in young clusters is negligible. Our findings disagree strongly with the recent claim from a microlensing study that free-floating planetary-mass objects are twice as common as stars–if the microlensing result is confirmed, those isolated Jupiter-mass objects must have a different origin from brown dwarfs and planemos observed in young clusters.
brown dwarfs - planets and satellites: formation - stars: formation - stars: luminosity function, mass function - stars: pre-main sequence
VizieR on-line data:
<Available at CDS (J/ApJ/756/24): table2.dat table4.dat table6.dat table7.dat tablen1.dat tablen2.dat izcat.dat jkscat.dat> (includes tables of 2012ApJ...744....6S)
Table 1: SONYC NGC 1333-NN (Nos 39-45) added. Table 2: [SJM2012] JHHMMSS.ss+DDMMSS.s N=12
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